Process of forming diffusion alloys on metallic refractory materials

ABSTRACT

THIS INVENTION IS DIRECTED TO A PROCESS OF FORMING A IDFFUSION ALLOY OF A SELECTED ADDITION METAL, SUCH AS TANTALUM, ON ANY ONE OR MORE DIFFERENT MTALLIC REFRACTORY MATERIALS. THE PROCESS IS EFFECTED THROUGH THE USE OF A REACTIVE MASS CONTAINING AN INERT POWDERED DILUENT, AN ALLOY POWDER AND A HALOGN OR HALOGEN-CONTAINING CONSTITUENT. THE SAME REACTIVE MASS MAY BE USED TO TREAT ANY OR ALL OF THE GROUP OF DIFFERENT METALLIC REFRACTORY MATERIALS. THE ALLOY POWDER CONTAINS THE DESIRED ADDITION METAL A, MODERATOR METAL SUCH AS NICKEL OR COBALT AND A REGULATOR METAL SUCH AS CHROMIUM.   D R A W I N G

May

1974' P. GALMICHE 3,810,782 PROCESS OF FORMING DIFFUSION ALLOYS ON METALLIC REFRACTORY MATERIALS Filed March 1, 1972 2 A Tia.

a Hf 1'5 27] 0 At. wt. 0 I W United States Patent rm. on. case 9/02 US. Cl. 117-107.2 P 8 Claims ABSTRACT OF THE DISCLOSURE This invention is directed to a process of forming a diffusion alloy of a selected addiiton metal, such as tantalum, on any one or more different metallic refractory materials. The process is effected through the use of a reactive mass containing an inert powdered diluent, an alloy powder and a halogen or halogen-containing constituent. The same reactive mass may be used to treat any or all of the group of different metallic refractory materials. The alloy powder contains the desired addition metal a, moderator metal such as nickel or cobalt and a regulator metal such as chromium.

RELATED APPLICATION This application is a continuation-in-part of my copending application Ser. No. 709,791, filed Mar. 1, 1968, now abandoned, and bearing the same title.

THE INVENTION This invention relates generally to new and useful improvements in the formation of diffusion alloys on the proximal surface portions of metallic refractory materials and particularly seeks to provide a novel process by which a diffusion alloy of a selected addition heavy metal may be formed on any one or more different metallic refractory materials selected from a group of diffeernt such metallic refractory materials through the use of a single reactive mass capable of causing the formation of the desired diffusion alloy of the selected addition metal thereon. The metallic refractory materials so treated may have any desired form, structure or dimensions and may include partly or completely fashioned pieces, or elements that are preformed or sintered from metallic powders or even such metallic powders themselves.

For the purposes of this description it will be understood that the expression metallic refractory materials refers to metallic materials whose base, or at least the major part thereof (55% or more), is constituted by at least one metal of low electropositivity (lower than 0.5) and having a high melting point (greater than 1400 C.) and an atomic weight between 50 and 200, or to alloys of metals satisfying such criteria. The metals involved may include, among others, iron, cobalt, nickel, molybdenum and tungsten.

Such metallic materials also frequently contain a smaller proportion of one or more metallic elements of high electropositivity (higher than 1) and relatively low atomic weight (lower than 60) that form additives. Among such additive forming elements are aluminum, titanium and vanadium.

Such metallic materials may also contain a refractory oxide, such as thoria, incorporated therein in the dispersed phase state.

ICE"

Further for the purposes of this description, the word additives will be understood to refer both to abovementioned additive elements and to the refractory oxides.

In general, it is recognized that the art of forming diffusion coatings or diffusion alloy coatings on metallic substrates is relatively old.

It is also recognized that this art also has developed to the stage at which various processes have been created by which a specified type of diffusion alloy may be formed on a specified type of metallic refractory material in order to increase its resistance to heat degradation or to increase the effectiveness of a subsequently applied protective coating, particularly where the diffusion alloy is formed through the use of at least one addition metal of high electropositivity (higher than 1), of a high melting point (higher than 1500 C.) and having an atomic weight higher than 90. These addition metals, for example, may include, hafnium, thorium and particularly tantalum, zirconium and niobium. Such processes generally have been effected by embedding the metallic refractory piece to be treated in a powder reactive mass comprising a powder of an alloy consisting of the addition metal and of a metal whose electropositivity is close to that of that metal (or metals) which is (or are) the essential constituent of the piece to be processed, thereby reducing the speed at which the addition metal deposits an inert diluent and a halogencontaining constituent, and heating same in a neutral or reducing atmosphere at predetermined ranges of times and temperatures.

It is reminded at this stage that the superficial diffusion alloys constituted by hinging heavy metals (whose atomic weight exceeds when used alone do not increase the high temperature corrosion resistance of the processed pieces. Such alloys are intended to provide an anchoring layer and to constitute a diffusion barrier offered to lighter protective metals-such as Cr and Al-which are later deposited and diffused by an adapted treatment. Due to their larger size, the heavy metal atoms do not substantially move, and consequently they oppose in depth diffusion of the protective metals during operation and increase the useful life of the processed pieces.

Even with the relative degree of success of these prior processes, it has been observed that difficulties are still encountered where such metallic refractory pieces are to be exposed, in service, to high temperatures and thermal shocks such as those to which turbine blades may be subjected. Such difficulties are even more apparent if attempts are made to employ a single reactive mass of a stated composition to form such a diffusion alloy of the selected addition metal upon more than one type of metallic refractory material in the same treatment at the same time.

For example, it has been observed that, when tantalum or another heavy metal exhibiting a high electropositivity has been the addition metal, unless the reactive mass is specifically formulated to be functionally complementary to the composition of the material to be treated, this addition metal has a tendency to form thick, irregular, fragile layers on the treated pieces that will readily scale off when the treated pieces are subjected to even small mechanical or thermal shocks.

It appears that this undesirable result is caused by an excessive rate or speed of deposition of the tantalum atoms on the metallic refractory material treated and to a too low rate or speed of diffusion of the tantalum atoms into the material.

The excessive speed of deposition of the tantalum appears to be due to its high electropositivity which causes a high reactivity with respect to the halogens taking part in the addition mechanism, and to a propensity to deposit rapidly on the lower electropositivity metals that constitute the base of the material being treated.

The low rate or speed of diffusion of the tantalum appears to be due to 1) the large size of the tantalum atom (high atomic weight) and (2) the presence of the metallic additive elements of higher electropositivity in the refractory material. In the absence of the inventive features of the process, there is obtained a layer which is brittle and prone to peeling due to excessive content of heavy metal rather than a homogeneous and ductile layer suitable as an anchoring layer and diffusion barrier. Such a brittle layer adversely affects the resistance to destruction of the superficial coating of Cr alloy or Al alloy subsequently deposited rather than acting as a diffusion barrier.

Therefore, one problem is to obtain a diffusion layer of a heavy metal of high electropositivity on a metallic refractory piece that is uniform, adherent and ductile through the concurrent reduction in the speed of deposition thereof onto said piece and an increase in its speed of diffusion into said piece.

A further problem is to obtain such a diffusion layer in such a manner that a single type of treatment using a single type of reactive mass, under stated operating conditions, may effect the formation of such a diffusion layer either upon composite pieces formed from two or more different metallic refractory materials, or simultaneously upon two or more pieces each formed from a different metallic refractory material, or upon any one or more pieces formed from the same metallic refractory material selected from a group of different metallic refractory materials. To state this problem differently, the method of treatment and the reactive mass used therein should be as nearly universal as possible so that it no longer will be necessary to tailor the treatment method and the reactive mass to the specific type of metallic refractory material to be treated insofar as any addition metal is concerned.

These problems are solved, in accordance with this invention and as will be hereinafter more fully described, through the use of a polyvalent reaction mass that gives satisfaction with any of the metallic refractory materials that are practically envisageable.

Therefore, an object of this invention is to provide a novel method for forming a diffusion layer or diffusion alloy layer of a selected addition heavy metal on or into one or more pieces of different metallic refractory materials, or on or into one or more pieces of a composite metallic refractory material in which the components thereof are included in the said group of different metallic refractory materials, or on or into two or more pieces of the same metallic refractory material selected from the said group thereof, said diffusion layer being homogeneous, ductile, adherent and adapted to operate as a diffusion barrier opposing further diffusion under service conditions of protective metals (such as Cr and Al) subsequently deposited.

Another object of this invention is to provide a method of the character stated in which the diffusion layer or diffusion alloy layer is effected through the use of a polyvalent reaction mass.

Another object of this invention is to provide a method of the character stated in which the polyvalent reaction mass includes an alloy powder, a powdered inert diluent and a halogenated constitutent.

Another object of this invention is to provide a method of the character stated in which the alloy powder of the polyvalent reaction mass is a powder of a ternary alloy and includes a heavy addition metal of high electropositivity, a moderator metal of low electropositivity, and a regulator metal.

A further object of this invention is to provide a method of the character stated in which the alloy powder contains for each atoms of addition metal, from 30 to 200 atoms of moderator metal and from 20 to atoms of regulator" metal.

A further object of this invention is to provide a method of the character stated in which the addition metal may be one or more of the metals selected from the group consisting of zirconium, hafnium, thorium, tantalum and niobium; the moderator metal may be selected from the group consisting of nickel, cobalt, molybdenum, tungsten and iron; and the regulator" metal may be selected from the group consisting of chromium and iron (if iron is selected as the moderator metal, this latter group is then limited to chromium).

With these and other objects, the nature of which will be apparent, the invention will be more fully understood by reference to the accompanying detailed description, the examples and the appended claims.

It will have been understood, from the above general description and statement of objects, that this invention primarily is directed to the formation of a diffusion alloy on or into the surface of any of several different types of metallic refractory pieces.

More specifically, the invention is directed to a process of forming on an additive containing metallic refractory material a diffusion alloy of an addition heavy metal of high electropositivity in which the material to be treated is embedded in an intimately mixed reactive mass containing (1) an inert powdered diluent, such as alumina, whose heat of formation is higher than that of the oxide of the addition metal, (2) a finely powdered alloy comprising the addition, moderator and regulator metals and (3) a halogenated constituent such as a halide of ammonium; and the reactive mass and its embedded material is housed in a partly gastight treatment container disposed within a reducing or non-oxidizing atmosphere and brought to a temperature ranging between 800 C. and 1250 C. (preferably between 900 C. and 1100 C.) for a period of time from a fraction of an hour to about 20 hours, depending on the temperature selected.

The condition to be fulfilled by the regulator and "moderator metals referred to above will be better understood with reference to the drawing, where the single figure is a diagram or chart illustrating the atomic weight and electropositivity of some metals suitable for that use. The addition metals which are envisioned are plotted on that chart and include Ta (at. wt.: 181, electropositivity: about 1.1), 'Nb (93 and 1.1), Zr (91.22 and 1.54) Hf (178.50 and 1.7) and Th (232 and 1.9). The corresponding domain on the chart is indicated as A but it should be understood that such domain is for explanatory purposes only. It may be noted at that time that Ta, Nb and Zr are particularly of interest, since they are the least costly. Ta may be used for parts of any size. Nb and Zr. are of particular interest for small size parts which are prone to distortion when heated, because the processing temperature is slightly lower than that with Ta, for the same layer thickness and duration.

Now, the following relationship should be fulfilled:

e$ f 1" where is the electropositivity of the moderator metal,

is the electropositivity of the regulator metal,

is the electropositivity of the addition metal As appears from the figure, Cr and Fe which may be used as regulator metals are in a domain B which is under the domain -A. Cr represents a relatively universal regulator in view of its intermediate electropositivity which is substantially higher than those of the essential constituents of the parts to be processed and those of moderators (as will be seen later) while remaining much lower than those of the addition metals.

Referring now to the moderators (domain C) Co, Ni and Mo can be used under all circumstances, regardless whether Cr or Fe is used as a regulator. Fe can also be used, but only if Cr (which is much more electropositive) is used as a regulator. This accounts for the overlap of domains B and C.

The powdered alloy in the reaction mass should comprise for each 100 atoms of the addition metal of high electropositivity (higher than 1), from 30 to 200 (preferably 50 to 100) atoms of moderator metal of low electropositivity (lower than 0.5) and having an atomic weight between 50 and 200, and from 20 to 140 atoms (preferably 60 to 100) atoms of regulator metal having an electropositivity of from 0.4 to 1.0 and having an atomic weight lower than 60.

The halogenated constituent of the reaction mass preferably should be other than fluorine.

The success of the method of this invention may be explained by the following description of the transfer mechanism, which is believed to be correct. For the purposes of this portion of the description, it will be assumed that the addition metal is tantalum, the moderator metal is nickel, the regulator metal is chromium, and the metallic refractory piece to be treated may be formed from any of those materials subsequently listed in Table I.

During the initial phase of the treatment, that is to say during the period of temperature increase, the tantalum (addition metal) contained in the prealloyed powder remains passive by reason of its low reactivity, at low temperatures, with respect to the halogenated constituents of the reactive mass.

Thus, in this initial phase of the treatment, only the chromium (regulator metal) contained in the prealloyed powder is attacked by the halogenated constituent of the reactive mass to give birth to a not very volatile chromium halide that only reacts above 750 C. on the superficial layers of the metallic refractory material undergoing treatment by slightly and uniformly increasing the chromium content of these superficial layers while eliminating the major part of the additives (of electropositivity higher than that of the tantalum) contained in these superficial layers.

Once this initial phase is completed, that is to say after uniformization of the composition of the superficial layers to be treated, both by the elimination of the troublesome additives and by the enrichment of these superficial layers in chromium, as the temperature continues to increase, the chromium halide created in the treatment atmosphere is progressively reduced by the tantalum (more electropositive than the chromium), which gives rise to the formation of limited quantities of tantalum halide assuring progressively the transfer of the tantalum to the treated material in conditions which are indeed those desired.

The nickel (moderator metal) contained in the prealloyed powder comes into play to reduce the speed of addition of the tantalum and to limit the concentration, in the superficial layers to a value which, in any case, cannot be greater than the concentration of tantalum in the prealloyed powder. Besides, the chromium comes into play to eliminate the major part of the troublesome additives of the superficial layers of the metallic refractory material. This preliminary elimination (associated with the superficial enrichment in chromium) favors an increase of the speed of diffusion of the tantalum and moreover considerably attenuates the differences of composition between the superficial layers of the treated materials, thus permitting the reactive mass to work in satisfactory conditions, whatever he the composition of the metallic refractory material treated.

Thus, the principal feature of the invention results in a reactive mass which, not only is polyvalent, but also permits layers of addition metals to be obtained that are non-porous, ductile, of regular thickness and non-fragile, due to the fact that it retards the deposition of the addition metal and accelerates its diffusion. In particular, in this manner, perfectly regular layers can 'be obtained on pieces of complex shape.

-In the practice of this invention it is preferable to form the ternary prealloyed powder comprising the addition, moderator and regulator metals in situ by a preliminary or blank treatment under the stated operating conditions, but without any of the metallic refractory pieces to be treated being present. Here, the starting materials are (1) the addition metal in the form of a very fine commercial powder/particle diameters ranging from a few microns to a few hundredths of a millimeter, or a still finer powder obtained by magnesothermy, (2) the moderator metal in the form of a very fine commercial powder, and (3) the regulator metal in the form of a fine or ultrafine powder. If the regulator metal is chromium, the fine powder may be electrolytic chromium or the ultrafine powder may be magnesothermic chromium. The resultant in situ alloy is in the form of a fine powder.

The metallic refractory materials listed in Table I hereunder are among and represent most of the compositions of the types of such materials that may be successfully treated by the method of this invention. Both the commercial designation and composition of these materials are given for convenience of reference.

TABLE I B 1900: base nickel, carbon 0.1%, chromium 8%, cobalt 10%, molybdenum 6%, aluminum 6%, tantalum 4%.

Inconel 713: base nickel, carbon 0.14%, chromium 13%,

molybdenum 4.5%, aluminum 6%.

IN base nickel, carbon 0.17%, chromium 10%, co-

balt 15%, vanadium 1%, molybdenum 3%, aluminum 5.5%, titanium 4.75%.

A.T.G.RE: base nickel, carbon 0.1%, chromium 20%,

titanium+aluminum 1%.

EPD 16: base nickel, carbon 0.12%, chromium 6%,

tungsten 11%, molybdenum 2%, aluminum 6%.

HS 25: base cobalt, carbon 0.08%, nickel 10%, chromium 20%, tungsten 15%.

W1 52: base cobalt, carbon 0.5%, tungsten 11%, niobium +tantalum 2%, iron 2%.

SM 302: base cobalt, carbon 0.8%, chromium 22%, tungsten 10%, tantalum 9%.

TD. nickel: nickel+2% ultrafine thoria.

By way of illustration, several examples will now be given of polyvalent reactive masses permitting a diffusion alloy of tantalum, niobium and zirconium, as the metal of addition, to be formed on any type of metallic refractory material containing metallic additive elements, such as those represented by the foregoing list of metallic refractory materials. In the first of these examples, the moderator metal is nickel and in the other the moderator metal is cobalt.

EXAMPLE 1 Reactive mass No. 1: Percent by weight 7 EXAMPLE 2 Reactive mass No. 2: Percent by weight Inert diluent: finely powdered calcined alu- In forming each of the above indicated reactive masses, the four component powders thereof are intimately mixed, 0.8% by weight of ammonium chloride and/or ammonium bromide is added thereto, and the mixture is heated for six hours at 1080 C.1l00 C. in a partly gastight container in a reducing or non-oxidizing atmosphere, thus effecting the in situ formation of the prealloyed powder containing the addition, moderator and regulator metals.

Then, when either of the above-mentioned reactive masses is to be used to effect the formation of the desired diffusion layer of the addition metal on a metallic refractory piece, 0.4% by weight of ammonium chloride and/ or ammonium bromide (or 0.3% by weight of ammonium chloride and/or ammonium bromide and 0.1% by weight of ammonium fluoride) is first added thereto, after which the piece(s) of the selected metallic refractory material(s) are embedded therein within a partly gastight container and heated for six hours at about 1980 C. in a reducing or non-oxidizing atmosphere.

The thus treated piece(s) result in addition metal sheathings that are quite plastic (even when cold); are not porous, cracked, scaled or fissured; have a smooth clean surface; and are on the order of from 25 to 35 microns thickness.

EXAMPLE 3 Niobium is substituted for tantalum as the addition metal in the reactive mass No. l or No. 2. The Nb atomic percentage should be substantially the same as that of tantalum. Since the atomic weights are almost the same, this leads to the possibility of using the same percentage by weight.

The heating treatment is under conditions similar to those in Examples 1 and 2. However, a lower temperature is preferable (1050 C.-1070 C. rather than 1080 C.- 1100 C.). The alloys of Table I may be treated in that way.

EXAMPLE 4 Zirconium is substituted for niobium with the same percentage by weight and under the same conditions as in Example 3.

The reactive mass may be regenerated after each treatment (or after a plurality of treatments) and repeatedly used. Regeneration is for the purpose of adjusting the composition to the original percentages. Regeneration is without addition of moderator metal since that metal is selected for not depositing and there is no consumption thereof. On the contrary the used mass is depleted in addition metal and in regular metal to a lesser extent. As a consequence suitable makeup quantities of the addition and regulator metals are added to compensate for the quantities consumed by diffusion at the surface of the treated pieces in a proportion selected to take into account the differences in depletion. The replacement ratio of the addition and regulator metals for this purpose is for instance about to 1 by weight when tantalum and chromium are used. Both metals are introduced as a Ta-Cr alloy in powdered form into the reactive mass.

When the reactive mass is heated again during a subsequent treatment, the halogen bearing vapors selectively attack the fresh alloy powder free of moderator. The resulting vapors of chromium and tantalum halogenides are trapped by the grains of ternary alloy powder and enrich it to its initial composition.

Generally, the pieces treated in accordance with this invention have been further treated physically or thermochemically by known processes so that final superficial layers of aluminum and alloys of aluminum and chromium are formed thereon; and in every instance where the pieces first have been treated in accordance with this invention, their resistance to mechanical, thermal and corrosive degradation has been greatly improved in a manner of kind rather than degree.

The advantage of this process using a single polyvalent mass to form a diffusion alloy on any selected one or more of several different or differently composed metallic refractory materials will be further appreciated when it is understood that, in all prior known processes for this purpose, it has been necessary to use a single reactive mass of specific composition for each different type of metallic refractory material.

Thus, the method of this invention now enables a metallurgical shop to diffusion alloy finish or refinish different pieces of different types of metallic refractory materials (turbine blades of different compositions, for example) without having to prepare a different reactive mass for each different type of metallic refractory material. Any excess of reactive mass from one treating run can be used to form the same type of diffusion alloy of the selected additional metal either on a small number of the same type of pieces on a subsequent treating run or on a small number of different types of pieces.

It is of course to be understood that variations in compositions of the reactive masses, the operating condition and other variables may be made within the scope of the appended claims.

I claim: 1. In a process of forming a diffusion alloy on at least one metallic refractory material comprising the steps of: embedding said material in a reactive mass containing, intimately mixed, a finely powdered inert diluent, a very fine alloy powder, and a halogen constituent;

housing said material and the reactive mass in a treatment chamber having a reducing or non-oxidizing atmosphere and raising the temperature of said material and mass to from about 800 C. to about 1250 C. for about a fraction of an hour to about 20 hours;

said very fine alloy powder comprises at least one addition metal selected from the group consisting of zirconium, hafnium, thorium, tantalum, and niobium, at least one moderator metal selected from the group consisting of nickel, cobalt, molybdenum, tungsten, and iron, and at least one regulator metal selected from the group consisting of chromium and iron or consisting of chromium in the event that the moderator is iron, the relative proportions of the addition, moderator, and regulator metals being such that for every atoms of addition metal, there are from 30 to 200 atoms of moderator metal and from 20 to atoms of regulator metal.

2. The process of claim 1 in which said metallic refractory material is selected from the group consisting of an alloy of nickel, an alloy of cobalt, an alloy of chromium and nickel.

3. The process of claim 1 in which said very fine alloy powder is formed in situ in said reactive mass prior to the time at which said piece of metallic refractory material is embedded therein by intermixing, in powdered form, said inert diluent, said addition, moderator and regulator metals and the said halogen constituent, housing said mixture in a treatment chamber, and raising the temperature of the contained materials to from about 800 C. to about 1250 C. for from about a fraction of an hour to about 20 hours.

4. The process of claim 1 in which said temperature is References Cited inhegfangehof 1050 C.1100 C. and said time is On the UNITED STATES PATENTS or er SIX ours.

5. The process of claim 3 in which said temperature is a the ran e of1050 0-1100 C. and said time is on the 5 J 117 17'2 m 3,415,676 12/1968 NlShl et a]. 117-1071 order of SIX hours.

6. The process of claim 1 in which said addition metal FOREIGN PATENTS is tantalum. 1,482,827 6/ 1967 France.

7. The process of claim 1 in which said addition metal 1,15 6/ 9 9 Gr t ta is niobium.

8. The process of claim 1 in which said addition metal ALFRED LEAVITT Primary Examiner is zirconium. J. W. MASSIE, Assistant Examiner 

